Why is the Z-seam always on the outside?

[T9_AIR 0]

Whenever I print a round object, such as a ball, the z-seam always ends up on the outside wall, and never on the inside. From what I have seen, the seam must end up on a vertex, so if you have a box, why can't the seam be on the inner corner? 

[GregValiant 1,410]

The seam is a function of FDM printing.  There will be a point on the outer wall where the nozzle starts to extrude it.  The Nozzle travels around extruding the outer wall, and then ends right where it started.  Those starts and ends are what makes the seam visible.

 

There is an Inner-Wall seam, a Support seam, an Infill Seam, a Prime Tower seam, and a Support Interface seam.  Even an "Ooze Shield" would have a seam.  Only the Outer Wall seam is displayed because it's the only one visible on a finished print.  The preview would get kind of cluttered if they were all shown.

 

 

 

 

[T9_AIR 0]

Everything I have seen online says that the Z-seam is caused by the wait before the printer moves up a layer. 

Quote

A Z seam forms during the time it takes the printhead, specifically the nozzle, to jump to the next layer. Right before this jump, extrusion is halted and the printhead stops moving for a split second while the Z-axis stepper motors lift it to the next layer.

During this pause, excess filament can ooze from the nozzle. This unwanted extrusion occurs due to built-up pressure in the hot end while printing. When the extruder briefly pauses, this pressure is released, causing a small outflow of material from the nozzle. The excess material gets stuck to the print at the pause location, creating the bumps of a Z seam.

https://all3dp.com/2/cura-z-seam-tutorial/

[Slashee_the_Cow 481]

23 minutes ago, T9_AIR said:

Everything I have seen online says that the Z-seam is caused by the wait before the printer moves up a layer. 

https://all3dp.com/2/cura-z-seam-tutorial/

+1 to my untrusted sources list.

 

I actually tend to get an underextruded Z seam, which is why the Outer Wall Wipe Distance option exists. The printer doesn't wait before it moves up a layer - it just moves up.

My Ender-3 V3 SE has a max Z feedrate of 10mm/s. Now, I'll admit I can't do all the maths regarding acceleration and such, but at 10mm/s, moving up a 0.2mm layer takes... less time that it took me to type that "...".

It'll take a bit longer if you have Retract at Layer Change turned on, but retracting and then priming 0.8mm at 40mm/s takes... yep, still less time than that "...".

 

Now let's look at if I print multiple objects all at once:

Each object has its own Z seam, every layer. Because each object has a place where it starts printing the outer wall every layer. With multiple objects like that, if the nozzle drools a bit, it'll give me stringing, but that's only because of the time it takes to move from one object to the next.

 

Next test:

That's 0.05mm layers (not that I could do that on my 0.4mm nozzle, but this is for demonstration purposes) at my printer's maximum build height. Total layers: 4995. How long will it take to print?

How much longer will it take if I lower the maximum Z feedrate from 10mm/s to 1mm/s?

*/me breaks out calculator*

Okay, so that's added... 48 milliseconds to each layer change.

How much do I extrude in 48ms? Well, since I'm printing at 60mm/s...

*/me breaks out calculator again*

It's equivalent to moving another 2.9mm...

*/me breaks out calculator again*

Ugh, geometry was never my speciality. Pretty much lacking spatial reasoning doesn't help. But that is, if my calculations are correct, about 0.058mm³ of filament. Or adjusting the calculations to print 0.2mm layers instead of 0.05mm, still only 0.23mm³ of filament. Which is really friggin tiny. That's how much is added if I lower the Z speed of my printer by 90% and print thousands of layers.

 

Okay, time for another theoretical experiment: what if I lower the Z acceleration from 500mm/s² to 1mm/s², while keeping the max feedrate at 1mm/s. How much does that add to the print time?

A bigger difference!

204.2ms per layer! A time discrepancy you might actually notice. And rather than run all my calculations again, I can just multiply the results by 4.25 (divide this time by the previous one). That would give us a blob of 0.98mm³ of filament. That's probably a noticeable blob.

 

But it's also a "worst case scenario" so bad that you're far better off buying a new printer than dealing with Z seam blobs on this theoretical printer.

[GregValiant 1,410]

@T9_AIR -

"...A Z seam forms during the time it takes the printhead, specifically the nozzle, to jump to the next layer..."

That is (at best) misleading.

Consider a hollow box with 5mm walls.  There is an outer wall facing inward, and an outer wall facing outward.  Whichever outer wall is printed first, there is no layer change when moving to the other outer wall and yet each outer wall will have a Z-seam.  The layer change doesn't come into it.

1. In FDM printing there is always a Z-seam.
2. If you can stick the Z-seam in a corner it's less noticeable.
3. See #1.
4. If the object is round you have two choices:
   1. Make the Z-seam location "Random".  There is still a Z-seam (see #1) but it is at a different location on every layer so it doesn't form a line.  This works well on things like printed threads.  It can often look like randomly located zits on a print.
   2. The Z-seams will all be lined up somewhere on the print.  It is the customary scar.
5. See #1.

This is two identical parts.  You can see that the inner diameter has a Z-seam and the outer diameter has a Z-seam.

 

Edited May 3 by GregValiant